1. Field of the Invention
This invention relates to a cylindrical winding used for induction electrical apparatus such as transformers, or more in particular to a multilayer winding in which the length of the oil gaps formed between adjacent layer windings is adjusted to reduce the electric field intensity at the edges of the layer windings.
2. Description of the Prior Art
It is well known that the high-voltage winding of such induction apparatus as transformers is subjected to oscillation of electrical potential when impressed with an impulse voltage such as lightning surge and switching surge which, in an extreme case, leads to the dielectric breakdown of the inductor involved. In order to improve the impulse voltage characteristics of the winding, it is common practice to arrange the winding to have larger series distributed electrostatic capacitance than the distributed earth electrostatic capacitance to achieve a substantially uniform, or linear potential distribution over the whole winding from one end to the other thereof.
Generally, when an impulse voltage is applied to a line terminal of the high-voltage winding, the initial characteristics of potential distribution due to such an impulse voltage are expressed as a function of .alpha. = .sqroot.Cg/Cs, where Cg is the earth electrostatic capacity of the winding and Cs is the series electrostatic capacity of the winding. As is well known, the potential distribution of the winding is more uniform the smaller the value of .alpha..
Typical high-voltage winding construction of transformers takes the form of interleaved disc windings or concentric cylindrical windings. It is also well known to those skilled in the art that, because of easiness in obtaining larger series electrostatic capacity of the winding, the cylindrical winding construction comprising a multi-layer winding is preferable, as will be described more in detail later.
The cylindrical winding assembly comprises a plurality of layer windings each of which is made by coiling a conductor in a suitable number of turns depending on the current capacity of the conductor and in a cylindrical form. The conductor may be a single flat-type wire with suitable insulation covering or a transposed wire made from a bundle of enamel-covered narrow flat-type strands twisted and collectively insulation-covered. The layer windings are made in cylindrical form with different diameters and arranged concentrically with each other. The conductor of each layer winding is connected at one end thereof in series with the conductor of an adjacent layer winding at the top or bottom thereof. The open end of the conductor of the innermost layer winding and the open end of the conductor of the outermost layer winding are connected respectively with neutral and high-voltage bushing terminals.
In such a cylindrical winding assembly, the series electrostatic capacity Cs becomes larger, for the reasons as will be described hereinunder, thereby to reduce the value .alpha. which determines the initial characteristics of the impulse voltage on the windings. This is because a cylindrical winding assembly comprising a plurality of layer windings satisfies (a) and (b) of the following conditions for increasing the series electrostatic capacity of the winding assembly: (a) to increase the area of the surfaces of respective windings facing each other, (b) to decrease the distance between adjacent windings, and (c) to enlarge the charge voltage between adjacent windings. The cylindrical winding assembly, therefore, develops greater resistivity against impulse voltage than the disc winding assembly.
In spite of this advantage, the cylindrical winding assembly has the following problem: In what may be called an "oblique cylindrical winding assembly," in which the layer windings, some of which have a shape of frustum of a cone, are connected in series with each other at the top and bottom ends, alternately and shield rings are provided at the ends of the layer windings, provision is made to dispose layer insulators between the layer windings. One of the layer insulators which is disposed between the innermost layer winding and a winding such as the low voltage winding or the like is formed with a uniform thickness over the axial length thereof, while the remaining layer insulators which are disposed between the layer windings are formed with a cross-section in the form of wedge having tapered thickness such that the thicker portion of the layer insulator is disposed between the opposing portions of adjacent layer windings which are subjected to higher potential difference when energized. These layer insulators generally are made in such a manner that insulating paper such as kraft paper 0.1 to 0.2 mm thick is cut into a predetermined size and wound on each of the layer windings, when it has been formed, to shape according to the oblique winding method in which the insulating layer is thicker at one end than at the other end. One end of each layer insulator thus wound is bent so as to hang over the shield ring arranged at the ends of each winding for reducing the electric field intensity while at the same time maintaining a predetermined oil gap with the same, thereby forming a flange. Alternatively, the layer insulators may be combined with flanged insulating member, respectively.
In the conventional oblique winding method for making the layer insulators, the insulating material is wound so as to hold duct pieces axially of the winding between the layer winding and the formed layer insulator thereby forming therebetween an oil gap of predetermined size as oil path for cooling the winding. The width of the oil gap is uniform from the bottom to top of the winding, while the formed layer insulator is formed to have a wedge-shaped cross-section with a constant inclination, and therefore the layer insulator has, at the portion adjacent to the top of the winding, a larger thickness or higher dielectric strength than commensurated with the potential difference between adjacent layer windings. As a result, the potential difference is not uniformly shared by the layer insulator and the insulating oil at the end of the winding in the neighborhood of the shield ring facing the oil gap through the layer insulator and therefore the surface electric field at that part and the electric field of the oil gap itself have extremely increased intensity, thus causing partial discharge. In an extreme case, this may lead to a serious accident such as a dielectric breakdown.
Generally, the intensity E of a uniform electric field of the shield ring surface facing the oil gap is expressed as ##EQU1## where V is the potential difference, do the gap width, dp the thickness of the layer insulator, .epsilon..sub.o the dielectric constant of the oil, and .epsilon..sub.p the dielectric constant of the layer insulator.
Usually, the dielectric constant .epsilon..sub.o of oil is smaller than the dielectric constant .epsilon..sub.p of the layer insulator, and therefore, the increased thickness of the layer insulator more than required will cause an increased electric field intensity of the oil gap, resulting in a great disadvantage in insulation design, the distance between adjacent layer windings d(= do + dp) being constant.
The above-mentioned problem is not limited to the oblique cylindrical winding assembly but is encountered also by a typical cylindrical winding assembly provided with a cylindrical layer insulator arranged inside of each of the layer windings, which insulator has the same thickness over the entire axial length thereof and has flanges at the upper and lower ends thereof.